Declines in the productivity of oil and gas wells are frequently caused by the migration of fines toward the wellbore of a subterranean formation. Fines, which normally consist of minutely sized clay and sand particles, can plug and damage a formation and may result in up to a 20-fold, and at times total, reduction in permeability. Conventional sand control techniques such as gravel packing and sand consolidation are sometimes ineffective because fines are much smaller than sand grains and normally cannot be filtered or screened out by gravel beds and consolidated sand treatments are restricted to small vertical intervals. In addition, gravel packing and sand consolidation are normally confined to areas surrounding the immediate vicinity of the wellbore. Fines movement, however, can cause damage at points which are deep in the production zone of the formation as well as points which are near the wellbore region.
Normally, these fines, including the clays, are quiescent causing no obstruction to flow to the wellbore by the capillary system of the formation. When the fines are dispersed, they begin to migrate in the production stream and, too frequently, they incur a constriction in the capillary where they bridge off and severely diminish the flow rate.
The agent that disperses the quiescent fines is frequently the introduction of a water foreign to the formation. The foreign water is often fresh or relatively fresh compared to the native formation brine. The change in the water can cause fines to disperse from their repository or come loose from adhesion to capillary walls.
It is well known that the permeability of clay sandstones decreases rapidly and significantly when the salt water present in the sandstone is replaced by fresh water. The sensitivity of sandstone to fresh water is primarily due to migration of clay particles (see "Water Sensitivity of Sandstones," Society of Petroleum Engineers of AIME, by K. C. Khilar et al., (Feb. 1983) pp. 55-64). Based on experimental observations, Khilar et al proposed the following mechanism to describe the dependence of water sensitivity in sandstone on the rate of salinity change.
In the case of an abrupt decrease in the salt concentration, a large number of clay particles are released in a short time and consequently, the particle concentration increased rapidly. As the particle concentration increased, more particles are captured due to bridging or "log-jams" at the pore and thus permeability was reduced significantly. Conversely, as the salt concentration is decreased slowly, the clay particles are released over a longer period of time. The particle concentration remains lower and many clay particles do not arrive at the pore at the same time to cause a "log-jam" effect. Therefore, the number of clay particles captured is lower and the reduction in permeability is also lowered. Importantly, it has been shown that there exists a critical particle concentration above which bridging occurs (Khilar et al 1983).
Others have proposed various ways to prevent permeability reductions caused by clay migration or clay expansions. For example, Henderson, U.S. Pat. No. 4,031,959, suggested the use of an aqueous solution of alkaline earth metal hydroxides and a salt of an alkaline earth metal which was injected down a wellbore and into a hydrocarbon reservoir in volume quantities sufficient to fill the pore spaces of said reservoir to some distance from the wellbore. McLaughlin, U.S. Pat. No. 4,366,071 proposed the use of a class of organic polycationic polymer compositions for treating earthen formations such as oil wells to stabilize clay against dispersion and expansion due to water.
Watkins, U.S. Pat. No. 4,018,285 sought to control fines migration by contacting the formation with a dilute resin solution. The resin solution was displaced into the formation via a diluent causing the deposit of a thin resin film. Later the diluent was extracted and the resin cured to the infusible state. The preferred resin was a one-step phenolic type resin which cured at formation temperatures.
Until now, no one has combined the critical rate of salinity decrease with the critical fluid flow rate to control fines migration near the wellbore so as to increase the permeability of a formation and also to increase the production of hydrocarbons.